FOR THE STEAK:
Rinse steak and pat dry with a paper towel.
Rub oil on the steak. Season both sides of steak with salt
and black pepper.
Place a 12-inch cast-iron skillet over
medium-high heat for 2 to 3 minutes, or until hot.
Cook steak in the hot skillet 3 to 4 minutes on each side or
until a meat thermometer inserted into thickest portion
registers 130℉ (medium-rare).
Remove from heat and let stand for 5 minutes.
Cut steak diagonally across grain into ¼-inch-thick slices,
and place on a serving platter.
Pour chimichurri over sliced steak, and serve immediately with arugula, lemon, and chimichurri sauce.
Make chimichurri by stirring together garlic, a couple pinches of salt, a dash of red pepper flakes, and rosemary in a small bowl.
Stir in red wine vinegar; let stand 5 minutes. Stir in parsley.
Add remaining olive oil in a slow, steady stream, whisking constantly until smooth.